The catalytic reaction of SO.sub.2 with oxygen to SO.sub.3 in contact process plants is presently effected with the aid of catalysts which comprise V.sub.2 O.sub.5 and alkali sulfate as active substances. Such catalysts are required to effect a high conversion of SO.sub.2 to SO.sub.3 at a low reaction temperature and should not be susceptible to damage at elevated temperatures and should have a high mechanical strength. These properties depend largely on the manner in which the catalyst bodies have been produced.
The impregnation of prefabricated carrier bodies with a solution of the active substances affords several advantages over the use of catalysts which have been produced in a single stage, in which carrier substances and active substances are mixed and are subsequently shaped to form catalyst bodies.
The prefabricated carrier bodies usually consist of SiO.sub.2 and can be produced irrespective of the active substances so that carrier bodies can be made which have a consistent composition, a large internal surface area, and a high mechanical strength. The impregnation results in a uniform distribution of the active substances throughout the carrier bodies so that the resulting catalyst bodies have a uniform composition and varying distributions of impregnants will be avoided. The catalysts have high conversion activities.
The production of catalysts by an impregnation of prefabricated carrier bodies has been described in EP 0 047 540 (see U.S. Pat. No. 4,431,573). The impregnation is effected with an impregnating solution which per liter contains 600 to 1100 g 5H.sub.2 SO.sub.4 and 140 to 830 g V.sub.2 O.sub.5 and alkali sulfate and is at a temperature between 60.degree. C. an the boiling temperature of the impregnating solution that is employed. The impregnated carrier bodies are dried with agitation at a maximum drying temperature of 700.degree. C. The catalysts have very high conversion activities even at low temperatures and are mechanically stable even at high temperatures.
From EP 0 151 823 (see U.S. Pat. Nos. 4,680,281 and 4,766,104) it is known that the conversion activity of such catalysts produced in two stages can be further increased by activating the dried catalysts under oxidizing conditions at a temperature from 750.degree. to 1000.degree. C. Drying and activation, however, result in a decomposition of the sulfates so that sulfuric acid mists escape. Expensive measures must be adopted in order to avoid emissions of pollutants.
Published German Patent Application 3022 894 describes for the production of fluidized bed catalysts a two-stage process in which carrier bodies are employed which contain Al.sub.2 O.sub.3 not in excess of 4% and are impregnated with a weak sulfuric acid solution of vanadyl oxysulfate which contains tetravalent vanadium, and of potassium hydrogen sulfate. This is succeeded by a drying at temperatures from 150.degree. to 250.degree. C. The carrier bodies or the impregnated carrier bodies are hardened at temperatures not in excess of 600.degree. C., preferably between 200.degree. and 400.degree. C. In that process the drying and hardening of the impregnated carrier bodies will also result in a formation of sulfuric acid mists, particularly during the activation described hereinbefore.